In “Houben—Weyl, Methoden der organischen Chemie”, vol. VI/3, Oxygen Compounds 1, Part 3 [1965], page 207, a process is described to produce a mixture of methylal and methanol with a mass fraction of approximately 8% of the latter in an industrial process using iron trichloride as catalyst. Separation of the remaining methanol from this mixture is difficult as this composition corresponds to an azeotrope.
Several processes have been described in the literature to recover pure methylal from reaction mixtures also comprising methanol.
In a paper by Volkov and Ivanov (Vysokomol. Soedin. 8 (8) [1966], pages 1459 to 1461) and a further paper by Vinokurov (Nauch. Doklady Vysskei Shkoly Lesoinzhener. Delo. No. 4 [1958], pages 193 to 195), a purification process for methylal is described comprising reacting the methanol present with metallic sodium. According to Ullmann's Encyclopaedie der Technischen Chemie, third edition, vol. 3, page 15 et seq., methanol can also be removed by extraction with concentrated aqueous calcium chloride solution, and by subsequent drying of the methylal.
In US 2006/0129 000 A1, a process for the synthesis of methylal from methanol and formaldehyde is described where an additional extractant has to be used which is fed to the rectifying section of a distillation column, water or an aqueous formaldehyde solution being preferred. An additional extractive rectification step is then needed to remove the added water, ethylene glycol being mentioned as the product of choice.
In U.S. Pat. No. 6,015,875, a process from making acetals is described where a mixture of alcohols and aldehydes is fed into a reaction zone in a column, and a mixture of alcohol and acetal is collected from the head stream of the column. This reaction is not complete, and the recovered distillate comprising the acetal has still a large amount of unreacted alcohol, 26.5% in the example. This mixture is further concentrated in a second column, to yield an overhead containing mass fractions of 0.2% of dimethyl ether, 3.5% of methanol, and 95.5% of methylal.
U.S. Pat. No. 6,379,507 B1 relates to a process for producing methylal, where a distillation column is fed in different heights with the efflux of at least four solid acid-filled reactors, the reactors being fed with liquid collected in bottom of the distillation column. Despite the large efforts taken in apparatus and ancillary equipment such as pumps, counter-current feeding of aqueous formalin solution, and addition of defoamer to the top portion of the column, a mass fraction of methylal of not more than 98% was reached.
From CN 1015 7117 A, a method to treat formaldehyde-containing industrial waste water has been known which involves adding methanol to the waste water, transferring the mixture to a reactor while simultaneously adding catalyst, and separating the excess methanol in a rectification tower. This process is merely designed to remove formaldehyde from the water, nothing can be learned about the yield and purity of methylal which is separated in this process from water as a mixture of methylal and methanol.
In the U.S. Pat. No. 4,385,965, a process for recovery of methylal from methanol-methylal mixtures is described which involves two rectifying columns operated at different pressures, with the pressure difference being at least 8 bar (0.8 MPa). The second column is typically run at from 9 bar to 30 bar (0.9 MPa to 3 MPa), preferably at from 10 bar to 15 bar (1 MPa to 1.5 MPa); the need to employ high pressure columns should be avoided. This patent does not describe the separation of aqueous formaldehyde solutions, nor does it teach the formation of methylal. Specially prepared mixtures of methanol and methylal, in examples 1, 2, and 3, and in example 4, a mixture of water, methylal, methanol, and a minor quantity of methyl formiate (corresponding to a mass fraction of 0.8%. in the mixture) were used.
Additional auxiliary substances or special equipment have to be used in these processes which makes these processes complicated and expensive.
Methylal has gained interest as fuel additive, solvent, and as adjuvant in certain polymers.